This invention relates to a method for attaching a gas diffusion electrode used as an oxygen cathode in sodium chloride electrolysis using an ion-exchange membrane and for leading electricity therefrom and to a structure for leading electricity.
Systems of leading electricity from a gas diffusion electrode to a cathode current collecting frame that have conventionally been used are roughly divided into the following two methods.
1) In Case of Sheet-shaped Gas Diffusion Electrode
The outer perimeter of a gas diffusion electrode is adjusted so that the gas diffusion electrode may slightly overlap with the gasket seal surface of a cathode chamber frame or a plate-shaped cathode current collecting frame (also called a cathode current collecting pan). A whole electrolytic cell is assembled by bringing the outer peripheral portion of the gas diffusion electrode and the gasket seal surface of the cathode chamber frame or the cathode current collecting pan into contact, setting a gasket thereon, and clamping, whereby the contact area is also clamped, and electricity is led from the gas diffusion electrode to the cathode current collecting frame via the thus clamped contact area.
2) In Case of Cathode Current Collecting Frame/gas Diffusion Electrode Integrated Body
A sheet-shaped diffusion electrode is placed on a cathode current collecting frame having attached thereto a mesh (which has high electrical conductivity if made of a metal) for making a gas chamber in such a manner that the catalyst layer of the gas diffusion electrode may cover the surface of the mesh. The catalyst is sintered at a high temperature under a high pressure in a press to form a catalyst layer and, at the same time, to integrate the catalyst layer with the mesh for a gas chamber. Electricity is lead from the gas diffusion electrode directly to the cathode current collecting frame and the cathode chamber frame.
In either case, a flow of electricity from the cathode current collecting frame to the cathode chamber frame (cathode element) is secured by connecting the cathode current collecting frame and the cathode chamber frame by welding or by mechanically connecting using a volt or the like.
However, these conventional methods of attaching a gas diffusion electrode and of leading electricity involve the following problems attributed to the action and function.
1) Leading Electricity From the Outer Peripheral Portion of Gas Diffusion Electrode
In the case of a small-sized electrolytic cell, a suitable conducting contact area can be secured with respect to the reaction area so that the contact current density can be reduced, and the electrical contact resistance can be reduced. In the case of a practical electrolytic cell having a reaction area of about 3 m2, however, since a suitable conducting contact area cannot be secured with respect to that reaction area, the contact current density is increased, and the electrical contact resistance is increased. Further, in a large-sized electrolytic cell of which the side of the reaction area is 1 m or longer, the structural resistance of the conductor in the gas diffusion electrode becomes high. From these facts, operating economy is inferior. In addition, where the gas diffusion electrode has small strength, it may be fractured at the part pressed by the gasket, and oxygen and a caustic soda solution leak from the fractured part.
2) Integration of Gas Diffusion Electrode, Mesh Sheet and Cathode Current Collecting Frame
A practical electrolytic cell has a reaction area of about 3 m2. In order to integrate the gas diffusion electrode, the mesh sheet and the cathode current collecting frame, a huge pressing machine, a huge mold, and a huge heating apparatus are needed, which is not economical. The cathode current collecting frame is liable to thermal deformation upon pressing at a high temperature, meeting great difficulty in securing flatness precision. Supposing these members could be integrated with good precision, the integrated cathode current collecting frame whose reaction area is as large as 3 m2 is mechanically weak, i.e., so flimsy that it is extremely hard to handle in transporting from a pressing factory to the place of installation of the electrolytic cell. This problem is common to the above-described case of xe2x80x9cleading electricity from the outer peripheral portion of gas diffusion electrodexe2x80x9d.
In renewing the gas diffusion electrode, it is difficult to detach the gas diffusion electrode from the cathode current collecting frame. Therefore, the cathode current collecting frame and the mesh sheet must be exchanged in the renewal of the gas diffusion electrode, which is uneconomical.
In the case of a practical electrolytic cell whose reaction area is as large as about 3 m2, a huge pressing machine and a huge pressing mold are required for integrating the gas diffusion electrode with the cathode current collecting frame.
The present invention has been made in the light of the above-mentioned problems in conventional techniques. It is an object of the invention to provide a method of attaching a gas diffusion electrode and of leading electricity and to provide a structure for leading electricity, which fulfill the following six requirements.
1. To reduce the sized of a unit gas diffusion electrode to make production and handling easier.
2. To diminish the resistance of a structure of the gas diffusion electrode itself by the size reduction of the gas diffusion electrode.
3. To make it easy to attach the gas diffusion electrode to the cathode current collecting frame while reducing the electrical resistance of the connecting part.
4. To make a structure allowing only the gas electrode to be detached in renewal of the electrode.
5. To make a structure for leading electricity which allows the cathode current collecting frame and the cathode chamber frame to easily be assembled together and separated apart and also which minimizes the electrical resistance of a structure at the cathode current collecting frame.
6. To make it possible, taking advantage of the separability, to carry out an integrating operation even in the production of a current collecting frame/gas diffusion electrode integrated body by using an exclusive tool which structurally withstands pressing, and yet to remove the gas diffusion electrode together with a conducting rib in renewing the gas diffusion electrode while leaving the cathode chamber frame re-useful.
The present inventors have conducted extensive studies in order to solve the above-described problems. As a result, they have found that the above problems can be overcome by interposing a conductor comprising a metallic mesh or spongy processed material between catalyst layers or attaching a catalyst layer onto the conductor to make a gas diffusion electrode allowing the conductor having excellent conductivity to be exposed only at the outer peripheral portion thereof, and fixing the exposed metallic conductor to a cathode current collecting frame, which functions as a medium for leading electricity from the gas diffusion electrode to a cathode chamber frame, either by welding (such as spot welding or laser welding) or by inserting a wedge into a groove made at a prescribed position of the cathode current collecting frame.
It has also been found that the above objects are accomplished by attaching a conducting rib to the back of the cathode current collecting frame of the gas diffusion electrode, attaching a conducting receptacle to the cathode chamber frame of the gas diffusion electrode of an electrolytic cell at the positions mating the conducting rib of the back of the current collecting frame, and fitting the conducting rib on the back into the receptacle. The present invention has been completed based on these findings.
That is, the present invention includes the following aspects.
The present invention relates to a method for leading electricity from a gas diffusion electrode, in which a part through which electricity is led to a cathode element is constructed by
providing a conductor having excellent conductivity and comprising a metallic mesh or spongy processed material, which conductor is sandwiched between catalyst layers except for the outer peripheral portion thereof or onto which conductor a catalyst layer is provided so that the conductor is exposed only at the outer peripheral portion of the thus-prepared gas diffusion electrode, and
electrically connecting the exposed part of the conductor to a cathode chamber current collecting frame.
The present invention relates to the above-described method for leading electricity from a gas diffusion electrode, wherein the exposed part of the conductor is fixed by welding to the cathode current collecting frame acting as a conductor to a cathode chamber frame.
The present invention relates to the above-described method for leading electricity from a gas diffusion electrode, wherein the welding of the exposed outer periphery of the conductor to the cathode current collecting frame is carried out by applying a metallic cover material having excellent conductivity on the conductor to protect the conductor from damage during welding.
The present invention relates to the above-described method for leading electricity from a gas diffusion electrode, wherein a gap between gas diffusion electrodes where the exposed outer peripheries of the conductors are welded is sealed with a sealant to prevent a caustic soda solution from entering.
The present invention relates to the above-described method for leading electricity from a gas diffusion electrode, wherein the cathode current collecting frame has a groove projecting from a gas chamber side toward the cathode element side at a prescribed position, the exposed part of the conductor is inserted into the groove, and then a wedge is inserted therein to connect the gas diffusion electrode to the cathode current collecting frame.
The present invention relates to the above-described method for leading electricity from the gas diffusion electrode, wherein the sealant which is applied to the space above the wedge to prevent a caustic soda solution from entering through the gap comprises the same material as the catalyst layer of the gas diffusion electrodes.
The present invention relates to an electricity leading structure of a gas diffusion electrode, wherein a cathode current collecting frame forms a partition dividing a gas chamber on the gas chamber side of the gas diffusion electrode and has a conducting rib projecting outward on its back for leading electricity.
The present invention relates to the above-described electricity leading structure of a gas diffusion electrode, wherein a cathode chamber frame is a conductor and has a receptacle made of copper or brass at a position mating with the above-described conducting rib on the back of the cathode current collecting frame.
The present invention relates to an electrolytic cell having an electricity leading structure that is easily assembled and disassembled by inserting the above-described conducting rib on the back of the cathode current collecting frame into the above-described receptacle for the cathode chamber frame.
Alkali-resistant and highly conductive metals which are worked into a metallic mesh or spongy processed material which can be used as a conductor in the present invention include platinum, gold, silver, nickel, and the like. Silver and nickel are preferred from the standpoint of economy, and silver is the most preferred for its excellent conductivity.
In the present invention, the welding means for fixing the exposed outer periphery of the gas diffusion electrode to the cathode current collecting frame include spot welding, laser welding, and the like. Electricity can flow from the gas diffusion electrode to the cathode current collecting frame through the weld joint. The weld line should not cross the flow of gas supplied to the gas diffusion electrodes. If it crosses the gas flow, it will hinder the gas flow in the gas chamber. Gas is usually made to flow downward in the gas chamber (i.e., interstices in a mesh sheet), the weld line is vertical.
In the present invention, a mesh sheet of the gas chamber on the inner side of the gas diffusion electrode can be fixed by welding the gas diffusion electrode. This method is not significance so much where the mesh sheet is metallic, because it is possible to fix the mesh sheet to the cathode current collecting frame by welding, such as spot welding, laser welding, etc. Where the mesh sheet is made of a resin, fixing by welding is difficult. Further, since a resin-made mesh sheet is lightweight, the above-described method of fixing the gas diffusion electrode by welding is effective to stabilize the mesh sheet.
When the outer peripheral exposed conductor of the gas diffusion electrode is fixed to the cathode current collecting frame by welding according to the invention, it is preferred to put a cover material, such as a rod or thin sheet of metal, e.g., nickel, on the conductor in order to prevent damage to the conductor during welding.
Sealants which can be used in the present invention to seal the gap between adjacent gas diffusion electrodes, i.e., the upper part of the weld of the conductor or the upper part of wedge fixing the conductor to the cathode current collecting frame hereinafter described, for prevention of a caustic soda solution""s entering is not particularly limited, and any alkali-resistant sealant can be used. For example, high-performance sealants, such as synthetic rubber, synthetic resins, particularly modified silicone resins and thiocholic resins are preferably used.
The longitudinal size of the gas diffusion electrode can be equal to the height of the electrolytic cell. The transverse size is preferably in a range of from 400 to 300 mm, taking into consideration the structural resistance of the conductor of the gas diffusion electrode and ease in production and handling of the gas diffusion electrode. Accordingly, a plurality of unit gas diffusion electrodes having such a small width are connected to each other to construct the cathode of the electrolytic cell with ease.
In renewing the gas diffusion electrode, only the gas diffusion electrode can be replaced by cutting the outer peripheral exposed conductor of the gas diffusion electrode. There is no need to remove the whole cathode chamber frame. Exchange of electrodes can then be carried out by fixing a new gas diffusion electrode to the cathode current collecting frame by welding.
It is preferred that the cathode current collecting frame be in a plate shape so that it serves as a partition for making a gas chamber for the gas diffusion electrode. While having a plate shape, the current collecting frame desirably has a recess to provide a gas chamber. A current collecting mesh serving as a gas chamber spacer is provided on the gas diffusion electrode side of the cathode current collecting frame thereby to provide a gas chamber between the gas diffusion electrode and the cathode current collecting frame. The cathode current collecting frame has a conducting rib for leading electricity. Any metal having excellent conductivity can be used as a conductive material for the conducting rib with no particular restriction. From the economical consideration, copper or brass is preferred, which is the same as the material of the receptacle.